The invention relates generally to electrochemical machining. More particularly, the invention relates to an electrode, an electrochemical machining assembly and an electrochemical machining method for forming curved holes.
A specialized adaptation of electrochemical machining, known as shaped-tube electrochemical machining (STEM), is used for drilling small, deep holes in electrically conductive materials. STEM is a non-contact electrochemical drilling process that can produce holes with aspect ratios as high as 300:1. It is the only known method that is capable of manufacturing the small, deep holes used for cooling blades of efficient gas turbines.
The efficiency of a gas turbine engine is directly proportional to the temperature of turbine gases channeled from the combustor of the engine and flowing over the turbine blades. For example, for gas turbine engines having relatively large blades, turbine gas temperatures approaching 1500° C. (2,700° F.) are typical. To withstand such high temperatures, these large blades are manufactured from advanced materials and typically include state-of-the-art type cooling features.
A turbine blade is typically cooled using a coolant such as compressor discharge air. The blade typically includes a cooling hole through which the air passes. A further design advancement has been the addition of internal ridges in the cooling hole to effect turbulent flow through the hole and increase cooling efficiency. Cooling features within the hole such as turbulence promoting ribs, or turbulators, thus increase the efficiency of the turbine.
The cooling holes commonly have an aspect ratio, or depth to diameter ratio, as large as 300:1, with a diameter as small as a few millimeters. The turbulators extend from sidewalls of the hole into the air passage about 0.2 millimeters (mm), for example.
The method currently used for drilling the cooling holes in turbine blades is a shaped-tube electrochemical machining (STEM) process. In this process, an electrically conductive workpiece is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of drilling tubes, each of which are utilised to form an aperture in the workpiece. The drilling tubes function as cathodes in the electrochemical machining process, while the workpiece acts as the anode. As the workpiece is flooded with an electrolyte solution from the drilling tubes, material is deplated from the workpiece in the vicinity of the leading edge of the drilling tubes to form holes.
Existing STEM processes and assemblies are capable only of drilling straight holes. However, it would be desirable to have the capability to drill curved holes that would better conform to the workpiece geometry to enhance cooling of hot areas, such as gas turbine blade platforms. Accordingly, it would be desirable to provide an improved electrochemical machining assembly and method to form curved holes in electrically conductive workpieces.